The Type 44-BX Velocity Microphones (MI-4027-B, -D, -H, -J and -K) are high-fidelity microphones of the ribbon type that are specially designed for broadcast studio use. They are constructed to withstand mechanical shocks, and to retain sensitivity and frequency response regardless of changes in temperature, humidity, and barometric pressure. Their essentially flat frequency response (50 to 15,000 cycles) is suitable for reproducing both voice and music.
The moving element of the microphone is a thin corrugated aluminum ribbon suspended between the poles of a strong Alnico magnet. The moving air particles that constitute sound waves vibrate the ribbon in the magnetic field. This motion causes an alternating voltage to be generated in the ribbon, the amplitude of which is proportional to the velocity of the air particles. The output voltage and the electrical impedance of the ribbon are raised to a value suitable for transmission of the signal to an amplifier, by a transformer built into the microphone case. The transformer is well shielded against stray magnetic fields by multiple shields of mu-metal and copper.